Seed production in a dioecious subcanopy conifer tree Taxus baccata : the positive effect of neighboring males and female heterozygosity

Seed production is known to reveal high individual variation in tree populations, shaping regeneration and competition patterns. In isolated populations, individual variation in seed production drives genetic drift and inbreeding, increasing the risk of local extinction. In this study, we investigated the variation in seed production within a natural remnant population of a dioecious conifer, Taxus baccata. The observed individual seed counts were highly variable and showed excessive zeros. The observed zeros were well explained by the small trunk circumference and the absence of neighboring males. Non-zero seed counts increased with female trunk circumference, neighboring male circumference, and female heterozygosity. In addition, seed production decreased with elevation, presumably reflecting water availability at the site. We did not observe an effect of canopy closure but, since trunk circumference showed a strong relationship with canopy closure, light availability may indirectly influence seed production by determining resources available for reproduction. Overall, the study shows that seed production in yew is strongly pollen-limited. However, for similarly sized and pollen-supplemented trees, seed production may be further determined by an individual genotype and micro-environmental factors. Whether the observed variation in seed production drives density-dependent effects (e.g. mortality) in recruited seedlings remains a matter for future studies.